The invention relates generally to pneumatic tool controls and more particularly to pneumatic tool throttle controls for use with multiple, fixtured motors.
It is common production line practice to utilize a plurality of assembly tools such as pneumatic tools to tighten a like plurality of fasteners disposed in an array. For example, the four or five bolts which are utilized to fasten a motor vehicle wheel to a hub are arranged in an array such that a like number of pneumatic tools arranged in a similar array may be utilized to expeditiously tighten the nuts simultaneously. The pneumatic tools are secured to a fixture such as a flat plate which maintains their appropriate spatial interrelationship and permits the handling and positioning of the tools as a unit.
In the prior art, it was common practice to utilize a plurality of independently operating tools. Typically such tools incorporate a one-shot operating sequence, i.e., operation is commenced by activation of a control or the application of air pressure and the tools operate as long as either the control is activated and air pressure is maintained or until the tool motor stalls and shuts off. At this time, the control or air pressure must be released in order for the tool to reset and begin a new operating cycle. During tightening down of the fasteners in a typical operating cycle, it is clear that one fastener will be secured and the associated pneumatic tool will stall first, followed sequentially by tightening and stall of the remaining fasteners and tools. Returning to the vehicle wheel and hub example, it should also be clear that if the wheel is not tightly seated on the hub when the first fastener is tightened down and the motor stalls, subsequent tightening of the remaining fasteners may cause loosening of the first fastener. Should the dimensional shift between the components be several thousandths of an inch, the first fastener may ultimately be loose and free upon the wheel hub lug. Since the tool associated with this loose fastener has stalled and completed its one-shot operating sequence, the fastener will not be properly tightened. Such operation is manifestly unsatisfactory and has prompted other design approaches.
A further difficulty of both single and multiple fixtured pneumatic tools relates to failure to complete the operating cycle. In typical pneumatic tools, the throttle is a springbiased lever which the operator depresses to activate the tool and releases subsequent to the tightening down of the fastener which is typically manifested by stalling of the drive motor. For various reasons such as haste or inattention, it has been found that an operator may release the throttle lever prior to motor stall and leave the fastener torqued at some arbitrary, incorrect and deficient value. Typically, such occurrences are random, making them especially difficult and expensive to locate and correct.
One suggested response to these difficulties comprehends the use of air logic circuits which utilize signals from each motor to determine torque application and the ultimate stall conditions. Such logic circuitry tends to be not only expensive, but vulnerable to tampering and misadjustment.